Nice looking rock!!! I would add some more. Calcium based rocks are prefered for SW set-ups to help maintain PH and hardness. They are also more benificial to small SW critters and inverts including corals.
A couple of good things about the lava rock is it will be a good place for benificial bacteria to develop and it is low in silicates.
How did the bottom crack? You might be able to have it replaced under warranty if it was 90 days or less old (at least that's for All-Glass tanks) and wasn't your fault. You could also try and fix the tank.